I'm sure someone's done this before but here's a neat worldbuilding idea:

Humanity finally expands into the stars, only to find abandoned cities from a hundreds of thousands of species on all the worlds we visit. Some systems have half finished mega-structures, some have have barely entered the industrial revolution.

The only common factor is that all signs of habitation seem to have ended at the same time, adjusted for light delay.

Humanity is alone in the universe, and something happened to make it that way.

As humanity peers ever further into the lifeless void, we start to notice things. Long derelict world-ships on escape trajectories out of the galaxy, most having burnt through their fuel eons ago. We only found the first - the bussard scramjets - by virtue of their continued automated burns at low-g acceleration.

The first alcubierre warpship (or antimatter rocket, if you want to maintain the no-ftl rule) to arrive at the site of one of these world ships found nothing but voided computer banks and empty living quarters. The next, a sleeper ship, was found with similarly blank computers and millions of empty stasis pods.

On a few of these ships strange markings were found etched into the metal hulls - anything else would've degraded by now. These markings, presumably writing of some sort, have been wildly different on every ship. Only 1 thing remains constant, a single symbol with only minor variations.

This symbol appears to be that of a star.

Humanity doesn't know what to make of this at first, we already know that stars existed long before these vanished civilizations did. Perhaps they were longing for stars, after so long in the intergalactic void. Or perhaps there's something we don't yet know. We are eager for answers.

In a system devoid of planets and asteroids, a massive star lifting process is begun, one of many at this point, for the purposes of creating further dyson swarms.

But there's something different about this star. As matter is torn from it the space surrounding it warps and thrashes for several weeks, before suddenly ceasing. The scientists are puzzled, but eventually the process begins again.

Then, just as suddenly as the initial thrashing began, the stars go out. Expanding in a bubble at exactly the speed of light, the stars themselves flicker for a moment before plunging into darkness.

The first warpships to leave the seed system warn others, collecting a quickly growing armada of ships trying to escape the eternal night.

And thus begins the final 180,000 years of the galaxy - the final end of days.